1. Field of the Invention
The present invention relates to power components of vertical type capable of withstanding high voltages.
2. Discussion of the Related Art
FIG. 1 is a very simplified cross-section view illustrating the general structure of a conventional power component. This component is formed in a large silicon wafer and is surrounded at its external periphery with an isolating wall of a conductivity type opposite to that of the substrate. This isolating wall is intended to separate the component from other components on the same chip, or for creating an electrically inactive protection area at the border of a chip, where a cutting is performed with respect to a neighboring chip. More specifically, referring to FIG. 1, starting from an N-type substrate, a first manufacturing operation consists of forming from, the upper and lower surfaces of this substrate, deep diffusion regions 2 and 3 that join to form the isolating wall.
For practical reasons, the wafers cannot have thicknesses under 200 μm. Under this threshold, they would be likely to break too easily in handling associated with the manufacturing process. Thus, each of deep diffusions 2 and 3 must penetrate into the wafer by some hundred μm, for example, 125 μm for a wafer of a thickness from 210 to 240 μm, to ascertain that a continuous wall, sufficiently doped at the level of its median portion, is formed. This requires very long diffusions at high temperatures, for example 300 hours at 1280° C. Clearly, this operation must be performed on a silicon wafer before any other wafer doping operation. Otherwise, during this long thermal processing time, the implantations previously performed in the substrate would diffuse too deeply.
After forming the isolating walls, doped regions intended to form the desired vertical component, for example, as shown in FIG. 1, a thyristor, are formed. For this purpose, a P-type region 4 corresponding to the thyristor anode may be formed on the entire lower substrate surface, simultaneously to a P-type region 5 corresponding to the cathode gate region of this thyristor, on a portion of the upper substrate surface. Then, on the upper surface side, an N+diffusion is performed to form in region 5 a cathode region 6 and possibly, between region 5 and isolating wall 2, a peripheral channel stop ring 7.
As seen previously, the total thickness of the wafer is determined by manufacturing considerations, which are essentially mechanical. Further, the characteristics of P-type regions 4 and 5 are determined by the desired electric characteristics of the thyristor, which determine the doping level and the diffusion depth. Indeed, it is, for example, desired to have a sufficiently steep doping front between each of regions 4 and 5 and substrate 1 to improve the characteristics of the corresponding junctions, and especially to obtain a good injection characteristic of the PNP transistor at the level of the junction between substrate 1 and region 4.
Thus, in the case of the shown thyristor, if each of diffusions 4 and 5 has a depth on the order of 40 μm, and if the wafer has a 210-μm thickness, there will remain between P-N junctions 5-1 and 4-1 an area of substrate 1 having a 130-μm thickness. As is well known, this area of the substrate provides the off-state breakdown voltage characteristics to the power device. This area must thus be sufficiently thick. However, an excessive thickness of this area results in an increase of on-state losses of the power device. If a power device having a breakdown voltage on the order of 400 volts is desired to be obtained, it would be sufficient for the thickness of the region of substrate 1 to be on the order of 80 μm whereas, with the described manufacturing method, a thickness on the order of 130 μm is inevitably provided. No simple way of solving this problem is currently known. Indeed, increasing, for example, the thickness of layer 4 has the consequence that the junction profile of this layer risks not fulfilling the desired electric conditions.
More generally, the same problem is raised with any power device to be surrounded with an isolating wall, the rear surface of which is of a doping type opposite to that of a breakdown voltage substrate, for example a power transistor or an IGBT transistor.